1. Field of the Invention
This invention relates to a processing circuit and method for a liquid crystal display (LCD), especially to a fast-working residual display suppression circuit for LCD and a method thereto, suitable to be applied to a cellular phone.
2. Description of the Related Art
In a liquid crystal control panel circuit, this type ofs a Super Twiist Nematic (STN) liquid crystal panel, a voltage generation circuit produces different driving voltages (for example, V1, V2, V3, V4 and V5). The variation of a voltage source VDD and the different driving voltages at different times is used to create different brightness levels on a liquid crystal panel in order to present visual information. FIG. 1 depicts a schematic diagram of a typical LCD voltage generation circuit and a stable capacitor Cs thereof. In FIG. 1, each of the pixel driving voltages (V1, V2, V3, V4 and V5) is applied through a stable capacitor (Cs1-Cs5) to stabilize the respective driving voltage (i.e., for V1, V2, V3, V4 or V5) output. However, in the structure shown in FIG. 1, at the power source VDD powered off, these stable capacitors Cs can provide residual power to the circuit due to the residual charge, causing the display of a residual display until the residual charge sinks to a voltage low enough to influence on the visual effect. The residual display is more severe when the current LCD circuits are designed for low power consumption. In LCD power generation circuit design, there are two methods for this.
First, the LCD driving voltages V1, V2, V3, V4 and V5 are changed depending on the power source VDD for the digital control signal. In this type of design, the power-off speed has a positive orientation to the power-down speed of VDD, as with the timing shown in FIG. 2. During the period of tH, the VDD level is gradually decreased by discharge from the power-off so that the voltages of the column and row terminals SEGs and COMs on the LCD panel and the driving voltages V1, V2, V3, V4 and V5 change to 0V. The digital control signals DIGs on the terminals SEGs and COMs will distort with respect to these decreased voltages. Therefore, using this type ofn LCD, for example, Solomon""s LCD driving IC, may create the residual display due to control signal distortion, as well as its driving voltage changing with VDD. Accordingly, an extra voltage regulator is used to eliminate the change of the driving voltages V1-V5 influenced by the VDD variation.
Second, the voltages V1-V5 are designed to respectively maintain a fixed voltage difference to the voltage VDD when the VDD is over a desired operating voltage. For example, in Epson""s LCD driving IC, a voltage regulator circuit 31, as shown in FIG. 3, is added to maintain the signals at power-off. In FIG. 3, the voltage regulator circuit 31 includes an amplifier OP, an adjustable stable reference voltage device REF and an adjustable gainer. The amplifier OP controls the output voltage V5, using the reference voltage level from the device REF and the gain factor from the adjustable gainer consisting of the resistors R1 and R2, to a fixed difference between the voltages V5 and Vdd. At power-off, this type of LCD power voltage generation circuit can maintain the voltages V1-V5 by a voltage regulator. At this point, the residual display will become more severe if no any control signal is added. Therefore, an added control signal is critical for this type of circuit at power-off, so that the circuit has a period of time tH to discharge the voltages V1-V5 and clear the input of the signals SEGs and COMs. At this point, because the voltage VDD still exists (i.e., the substantial power-off is not complete), all digital control signals DIGs can function normally. The duration of the power-off for VDD is tR. As such, the signal will not distort after the power-off. However, the response time is longer, by tR+tH, compared to the first.
Accordingly, another circuit is shown in FIG. 5. In FIG. 5, the circuit can quickly avoid residual display without additional response time. As shown in FIG. 5, the circuit detects the start signal by a detection circuit 51. When the circuit (PD=Low) is started and the respectively stable capacitors with respect to the voltages V1 to V5 are charged completely (Start_up=Low), the detection circuit is started. After the start, a comparator 52 is used to compare a reference voltage VREF from a reference voltage source 53 and another reference voltage VSS in order to detect the power-off presentation. Another reference voltage can be a grounding voltage or a reference voltage input by a user. In the case of presenting the power-off and the lower reference voltage VREF than the reference voltage VSS, as shown in FIG. 6, when the VDD just begins to be pulled down, the difference of VDD to V1, V2, V3, V4 and V5, respectively, is maintained at a constant by the voltage regulator circuit. When the VDD is reduced to a certain level, a power-off action other than a noise presentation is detected. At this point, a one-shot control circuit 54 in FIG. 5 activates a discharge circuit 56 to release the LCD residual charge so as to eliminate the residual display. However, this is disadvantageous in that the discharge time is prolonged by continuously powering of the voltage V1 to V5 and discharge performance is poor when V5 greater than VSS.
Therefore, an object of the invention is to provide a fast-working residual display suppression circuit for LCD.
Another object of the invention is to provide a fast-working residual display suppression method, with both manual and automatic operability.
To realize the above and other objects, the invention provides a fast-working residual display suppression circuit for LCD and a method thereto. The fast-working LCD residual display suppression method includes the following steps. A control signal is received. If the control signal is an automatic control signal, a first reference voltage with a fixed difference to a supply voltage is compared to a second reference voltage according to an externally input start signal. As the first reference voltagexe2x89xa6the second reference voltage, an activating signal is output to an LCD power controller and a fast discharge circuit to quickly eliminate LCD residual display. Also, if the control signal is a manual control signal, the activating signal is directly output to drive the LCD power controller and the fast discharge circuit to quickly eliminate LCD residual display. As such, according to the activating signal, the LCD power controller cuts off the power supply to the fast discharge circuit and combines two discharge paths of the fast discharge circuit to produce an optimal discharge path to speed the discharge rate.
The fast-working residual display suppression circuit for LCD includes: a manual selector for outputting a manual control signal; an automatic detection circuit for automatically detecting the power state and output an automatic control signal; a selection switch for eliminating residual display and outputting an activating signal according to either the manual signal or the automatic signal; a signal level conversion circuit for receiving the activating signal and producing the desired voltage level output; a fast discharge circuit for quickly eliminating LCD residual display based on the activating signal and the desired voltage level output; and an LCD power controller for cutting off the power supplied to the fast discharge circuit based on the activating signal to speed up the discharge rate.